helioseismology

The branch of asteroseismology which deals with the pulsations of the Sun.
An observational study of the solar five-minute variability was
first published by Robert Leighton, Robert Noyes, and George Simon in 1962,
though the nature of this variability was not understood at the time. The
five-minute variability is now understood to be thousands of superimposed
acoustic waves passing through the solar interior -- the Sun is ringing
like a giant bell. The field of helioseismology has flourished from the
1970s onward, and is still a topic of ongoing research and debate.

Helioseismology has been successful for several reasons. For one,
the Sun is so close and so bright that we are able to obtain data with very
high signal to noise ratio, and can thus determine the spectrum of
oscillations with high accuracy. Furthermore, the Sun is a resolved object,
which means that we can observe oscillation modes of high angular degree,
l, unlike unresolved stars where we can only observe modes
of up to l = 2 or 3. Both of these facts combined means that we can observe
thousands of independent pulsation modes. These modes allow us to precisely
constrain the structure of the solar interior.

There are many things that helioseismology can reveal about the Sun. For
example, it allows us to determine the radius of the base of the
convection zone. It
allows us to determine the chemical composition of the solar envelope, and
study the effects of diffusion, rotation, and turbulence on the Sun's
interior structure. Precise knowledge of the Sun's structure also provides
a calibration point for studying other stars, particularly in the development
of computer simulations for studying stellar evolution. It also allows us
to determine equation of state and opacity data for stellar plasmas, again
useful for studying other stars. The primary failing of helioseismology is
that we have yet to observe oscillation modes that sensitively probe the
core of the Sun. For that, we rely on neutrino observations and
extrapolation of current data on nuclear reactions and the equation of
state at high temperatures and densities.

Several observational studies of the Sun are ongoing. The Solar and
Heliospheric Observatory or SOHO satellite has been observing the
Sun from the Earth-Sun L1 Lagrange point in space since early 1996, and
the GOLF (Global Oscillations at Low Frequencies) and MDI/SOI
(Michelson Doppler Imager/Solar Oscillations Investigation) instruments on
board are
dedicated to helioseismology. On the ground, the Global Oscillation Network
Group (GONG), LOWL (for "Low-l"), and Birmingham Solar Oscillations
Network (BiSON) have been observing the Sun for many years as well.

Sources: the SOHO website at http://sohowww.nascom.nasa.gov,
a good helioseismology summary at
http://soi.stanford.edu/results/heliowhat.html, and piles of papers
on my desk.